The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Many semiconductor processes require silicon oxide (such as SiO, SiO2, SiOxHy) to be formed as a film or deposited layer on a substrate. Methods for forming silicon oxide may include chemical vapor deposition (CVD) (such as thermal or plasma enhanced CVD, high density plasma (HDP) CVD). However, some applications such as pre-metal dielectric (PMD), interlayer dielectric (ILD) or shallow trench isolation (STI) require high aspect ratio filling. As aspect ratios increase, filling gaps using these CVD approaches becomes more difficult.
Flowable materials such as flowable oxide, spin-on dielectric (SOD), spin-on glass (SOG) and/or spin-on polymer (SOP) may also be used. Flowable materials tend to have good gap-filling properties, which are suitable for high aspect ratio applications. After application, the deposited layer undergoes further processing to convert the deposited layer to a high density dielectric and/or to convert the deposited layer to silicon oxide. The flowable materials also generally need to have film properties that match HDP oxide (e.g., low wet etch rate ratio (WERR) (such as less than 1.2:1 or 1.5:1 compared to thermally grown SiO2) and high density). For example only, the SOD may include polysilazanes (PSZs) and the SOG may include siloxanes, silsesquioxanes, and silazanes.
For STI applications with relatively high thermal budgets, conversion of the deposited layer may be done at high temperatures in an oxidizing atmosphere (typically oxygen or steam). When the oxidizing atmosphere is oxygen, thin crust-formation may occur and poor quality film usually resides below the thin crust layer. While steam tends to have improved oxidative and penetrative properties as compared to oxygen, oxidation of underlying silicon may occur in applications without a silicon nitride (SiN) liner. High temperatures in an oxidative atmosphere may not be used for certain applications having lower thermal budgets, which are generally specified by a period at a particular temperature. For example only, some PMD applications have thermal budgets of 400° C. or lower for a particular period.
Conversion of the deposited layer to a dense oxide at lower temperatures can be challenging. For example only, steam annealing at 400° C. or lower does not typically result in full conversion to oxide, even after long annealing periods such as 30 minutes. In addition, the quality of the oxide that is formed is usually not acceptable due to the presence of silanols (SiOH), and consequently the oxides have low density and high WERR.
Sub-atmospheric chemical vapor deposition (SACVD) processes may be used to deposit the oxide. Low temperature oxides may also be deposited using a variety of other techniques such as plasma enhanced CVD (PECVD), atomic layer deposition (ALD), and plasma enhanced ALD (PEALD). These approaches generally cannot fill reentrant structures adequately. These approaches may also have significant silanol content after conversion, high WERR and/or low density, which may require a high temperature annealing step that exceeds the thermal budget to fix.
Further processing may also be required after conversion to reduce silanols (SiOH) and/or to increase the density of the deposited layer. The process used to increase the density needs to be within the thermal budget of the application. One approach involves annealing the deposited layer at the highest temperature and longest period allowed by the thermal budget. For example for STI gap-fill applications, higher temperatures such as 700-800° C. for a particular period are allowed (but preferably not in an oxidizing atmosphere). In such a case, the deposited layer is annealed to drive out the silanols and further increase the density of the oxide. In applications where the thermal budget is 400-480° C. for a particular period, annealing has a very limited impact. Annealing can potentially lower the free OH in the deposited layer, but densification and silanol removal typically do not occur at these lower temperatures.